The present invention relates generally to acoustic transducers, and, more particularly, to motor assemblies for microphones to reduce parasitic capacitance.
Transducers and particularly microphones are typically utilized in hearing-aids. Generally, electret transducers comprise a casing having an opening which communicates with the interior of the case. An electret assembly including a diaphragm adjacent a charged plate having an electret material formed thereon is mounted within the case to form acoustic chambers on opposite sides of the diaphragm.
Acoustic signals enter one of the acoustic chambers allowing the diaphragm to respond thereto. Air pulsations created by the vibrations of the diaphragm pass from one acoustic chamber to the other acoustic chamber.
The electret material on the charged plate is connected to suitable electronic circuitry to permit electroacoustical interaction of the diaphragm and electret material on the backplate to provide an electrical signal representative of the acoustic signal. As is known, the converse operation may be provided by the transducer in that an electrical signal may be applied to the electret on the backplate to cause the diaphragm to vibrate and thereby to develop an acoustic signal which can be coupled out of the acoustic chamber.
In a transducer of the subject type, it is always a problem to reduce or minimize the parasitic capacitances, i.e. the capacitances that do not vary proportionally to the variation in the air vibrations but are stationary and are determined by the construction of the transducer. Specifically, in electret transducers and microphones, parasitic capacitances are present wherever the capacitance formed by the charged plate and the diaphragm cannot move under the influence of air vibrations. Typically, in the above-identified transducers parasitic capacitances are caused by the protrusions or bumps which maintain proper spacing between the diaphragm and charged plate.
Accordingly, an acoustical transducer in accordance with the present invention provides an inexpensive and simple solution to eliminate the drawbacks of the prior acoustical transducers.
The transducer of the present invention is adapted to provide an electret assembly, also referred to as a motor assembly, including a diaphragm, support member, and backplate which is simple and inexpensive to manufacture, and which provides a reduction in the fixed capacitance of the transducer. Generally, the motor assembly is located in a case to form acoustic chambers on opposite sides of the diaphragm. This type of transducer is suitable for hearing-aids, as well as for other uses.
According to one aspect of the present invention, the support member has a first side, a second side, and an aperture extending therethrough. A periphery of the diaphragm is connected to the second side of the support member such that a portion of the diaphragm is adjacent the aperture of the support member. The portion of the diaphragm that is not connected to the support member is capable of vibrating.
According to another aspect of the present invention, the backplate is mounted to the support member in a spaced relation. As such, the backplate is further spaced a distance from the diaphragm to provide a gap between the backplate and the diaphragm. Preferably, the entire backplate is spaced a distance from the diaphragm, enabling air movement between the diaphragm and the backplate and reducing unnecessary parasitic capacitance.
According to another aspect of the present invention, the backplate is charged. The charged material on the backplate cooperates with the vibrating diaphragm to develop a signal. An amplifier is electrically connected with a wire to the charged backplate. The wire allows the signal to be communicated to the amplifier which converts and amplifies the changes in capacitance into an electrical signal representative of those changes. The operation of the transducer is based on the change in capacitance between a fixed electrode, the backplate, and a movable diaphragm under the influence of external air (sound) vibrations. The change in this capacitance is proportional to the changes in air pressure and can be converted into amplified sound vibrations via the electronic amplifier described above.
According to yet another aspect of the present invention, a method for manufacturing the motor assembly is provided to attain accurate and proper spacing between the diaphragm and the backplate to reduce wasted output signal.
One object of the present invention is to provide a transducer motor assembly with a greatly reduced amount of parasitic electrical capacitance due to the elimination of support bumps to support the diaphragm and space the diaphragm from the backplate.
Another object of the present invention is to provide a transducer motor assembly which does not influence the transfer characteristics of the transducer.
Another object of the present invention is to provide a transducer motor assembly which does not waste potential output signal by having extra electrical capacitance in the transducer motor assembly, and which does not increase the noise level of the motor assembly.
Another object of the present invention is to provide a method for manufacturing such a transducer motor assembly which is efficient, inexpensive, and easily performed.
Other features and advantages of the invention will be apparent from the following specification taken in conjunction with the following drawings.